The ballot measure for the project required that the L.A.-to-San Francisco trip take no more than two hours, 40 minutes. Achieving that would mean building more viaducts and tunnels, which are costly.

California's proposed bullet train will need to soar over small towns on towering viaducts, split rich farm fields diagonally and burrow for miles under mountains for a simple reason: It has no time to spare.

In the fine print of a 2008 voter-approved measure funding the project was a little-noticed requirement that trains be able to rocket from Union Station in downtown Los Angeles to San Francisco in no more than two hours and 40 minutes.

It was an aggressive goal, requiring cutting-edge technology, and was originally intended to protect the sanctity of the bullet train concept from political compromise. Whether the California High Speed Rail Authority can meet such a schedule is far from certain. Even some backers of the project now say it was a mistake to lock in the strict requirement.

It's hardly an academic issue.

The need for speed is driving a number of environmentally difficult and extremely expensive design choices, contributing to the doubling of the project's cost to $98.5 billion. Pricey tunnels and viaducts would enable the train to run up to 220 mph, faster than most high-speed trains travel in Europe and Asia.

In addition to raising construction outlays, such velocity would increase electricity use sharply, working against another mandate, that the bullet train's revenues cover operating expenses. Costs of the project are expected to come under scrutiny Thursday at a Washington hearing held by the House Transportation and Infrastructure Committee.

Interviews with experts and a Times examination of the latest business plan for the project's urban and rural areas raise serious questions about whether the timetable can be met.

"I don't see how you can do it," said James Earp, a member of the California Transportation Commission and a union leader who helped orchestrate the 2008 ballot measure approving the project.

Rich Tolmach, director of the California Rail Foundation, which advocates for passenger rail projects, said design compromises to gain political support have added to the time the L.A.-to-San Francisco trip will take, leaving the system unable to meet the mandate.

The travel time limit also has become a legal weapon for opponents of the project. A lawsuit by Kings County and two local homeowners is seeking to halt construction partly by claiming that planners are violating state law because the train will be too slow.

Michael McGinley, a former commuter rail executive who as a private consultant advised the rail authority until last year, said the push to beat the clock is driving up costs beyond reason.

"The infrastructure gets progressively more complex as you move to higher speeds," he said. "It has been designed as an exercise in elegant advanced engineering without consideration of what makes sense as an investment."

The latest plans include major engineering changes. The system will need up to 168 miles of elevated viaducts, more than double the distance planned in 2009. Tunneling will increase more than 60% to 52 miles. The combined cost of viaducts and tunnels, which make up 43% of the system, has risen threefold to $34 billion.

Some state legislative leaders and rail authority officials say the time requirement never should have been put into the law. "It was a mistake," said Sen. Alan Lowenthal (D-Long Beach), a key supporter of the project who has asked increasingly tough questions about the cost.

Authority board member Dan Richard acknowledged that the high speeds are adding to the project's complexity and increasing costs. "Did it have to be that tightly stipulated? Probably not," Richard said. "You should not do engineering by ballot measure."

"If we are off [the time] at all, it is not a matter of that much," he added.

Will Kempton, a former California Department of Transportation director who heads a panel of experts who monitor the project for the authority, said he thinks the two-hour-and-40-minute time limit can be met.

Only nonstop trains between L.A. and San Francisco would make the fastest time. Many trains would stop at the more than half a dozen stations along the route, pushing their travel time well past three hours.

A half-dozen lawmakers who voted to put the project on the ballot said they didn't know the origin of the time limit.

Mehdi Morshed, the longtime chief executive of the rail authority who retired last year after a 30-year career promoting high-speed rail, told The Times it was his doing. "I am the one who insisted on putting the times in," he said.

"If we didn't do that, everything would be compromised and you would have a slow train to nowhere. All the revenue and ridership studies showed that for high-speed rail to be successful and pay for its operating costs, you have to be competitive with airplanes."

Getting a bullet train to run from Los Angeles to San Francisco in less than two hours and 40 minutes wouldn't be difficult if the route were straighter, Tolmach and others said.